This invention relates to a modulator and demodulator apparatus for modulating and demodulating transmission/reception data, and more particularly to a frequency offset dealing modulator and demodulator apparatus which allows removal and forecast of a frequency offset to reproduce an accurate receive signal.
FIG. 19 shows a general construction of an on-line system. Referring to FIG. 19, in the on-line system shown, a plurality of modems (modulator and demodulator apparatus) 203 are connected to a host computer 201 by way of a communication control apparatus (CCP) 202, and each of the modems 203 is connected by way of an analog circuit 204 to another modem 203' installed at another location. A terminal 205 is connected to each of the models 203'.
The on-line system further includes a network supervisory apparatus 206, for which a secondary channel is used.
By the way, a state signal of a modem can be transmitted, from each of the host side modems 203 shown in FIG. 19, as it is to the network supervisory apparatus 206, but from each of the terminal side modems 203', a state signal thereof is transmitted to the associated host side modem 203 so that it is transmitted by way of the modem 203 to the network supervisory apparatus 206.
Since a state signal of a modem must necessarily be transmitted without having any influence on main data, each of the modems 203 and 203' divides, for example, a voice band of 0.3 kHz to 3.4 kHz by frequency division to provide a secondary channel for secondary data in addition to a main channel for main data as seen in FIG. 20.
It is to be noted that phase shift keying (PSK), orthogonal amplitude modulation (QAM) or some other modulation is used for a main signal while frequency shift keying (FSK) is used for a secondary signal.
Meanwhile, a modem generates, upon transmission, signal points having a predetermined eye pattern (data point arrangement pattern on a phase plane) to modulate and transmit main data and secondary data and demodulates, upon reception, a receive signal to reproduce such main data and secondary data. In such a wide band modem as just described, a function for a frequency offset is not provided for both of the main channel and the secondary channel, but reproduction of an impulse which is used for initialization of an automatic equalization (AEQ) section provided at a next stage to an automatic gain control (AGC) section is performed by a sum circuit 100 provided on the output side of the AGC circuit 6 as shown in FIG. 21. In particular, referring to FIG. 21, the sum circuit 100 is constructed such that the AGC output (training pattern signal) of the AGC circuit 6 is branched into two signals and one of the two branched signals is delayed by a delay section 50 and then added to the other branched signal to reproduce an impulse. For example, when the transmission rate is 9,600 bps, the Baud rate of the main channel can take such a high value as 2,400 Bauds as seen from FIG. 22(a), and consequently, the signal points can be reduced to 16 values. Therefore, even if a frequency offset removal circuit, a frequency offset forecast circuit of the like is not provided specifically, since the distance between signal points is sufficiently large for an offset up to .+-.4 Hz or so as seen in FIG. 23, even if some deterioration of an eye pattern is involved, a sufficient follow-up to the offset can be provided by a CAPC circuit (carrier phase correction section).
However, with such a modem as described above, since reproduction of an impulse is performed without removing a frequency offset only using a sum circuit, discontinuity of a signal sometimes occurs.
By the way, it is required for modems in recent years to establish multiple point connection in addition to a rise of the communication rate to reduce the cost of the circuit. To this end, it is an effective technique to divide a frequency band of a main channel into a plurality of bands to transmit a plurality of data by way of the same circuit.
However, if a main channel is divided by frequency band division to transmit a plurality of data by way of the same circuit in this manner, then while the transmission rate is equal, the Baud rate per one channel decreases as seen from FIG. 22(b), and therefore, the number of signal points must be increased. Consequently, as the number of signal points increases, since the distance between signal points decreases as seen in FIG. 24, an influence of a frequency offset becomes significant although the RS-CS time (time for request-to-send to clear-to-send) is short.
On the secondary channel side, however, since only summing is involved, an accurate impulse cannot be reproduced. For example, when the modulation rate is 48 Hz and the frequency offset is 24 Hz, then the receive signal will rotate, and accordingly, even if summing is performed, an impulse thus reproduced will have some discontinuity of the signal.
Even on the main channel side, as the Baud rate becomes low, even if the offset amount is equal, the amount of rotation for one symbol will increase as seen from FIG. 25. As a result, the displacement of an impulse thus reproduced increases, and as the influence of the offset increases, it becomes difficult to follow up the signal, and consequently, it becomes difficult to calculate accurate tap coefficients of an automatic equalizer.